This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-169216, filed Jun. 16, 1999; and No. 11-173206, filed Jun. 18, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode-ray tube whose panel has a flattened effective section.
A commonly-used color cathode-ray tube includes a vacuum envelope having a substantially rectangular panel and a funnel. The panel has an effective section with a curved surface, and a phosphor screen is formed on the inner surface of the effective section. A substantially rectangular shadow mask is opposed to the phosphor screen. The shadow mask has a mask body in which a number of electron beam passage apertures are formed and a mask frame supporting the periphery of the mask body. The shadow mask is supported on the inner surface of the panel by fitting elastic support members, which are attached to the mask frame, to their corresponding stud pins provided on the panel.
An electron gun for emitting electron beams is arranged in a neck of the funnel. In this color cathode-ray tube, three electron beams, which are emitted from the electron gun, are deflected by a deflection yoke mounted on the outer surface of the funnel and scan the phosphor screen horizontally and vertically through the electron beam passage apertures of the shadow mask, thereby displaying a color image.
As the phosphor screen, a black stripe type is known in which a plurality of strip-shaped, black light absorption layers, which are long in the short-axis direction of the panel, are arranged in parallel in the long-axis direction thereof, and strip-shaped three-color phosphor layers, which are long in the short-axis direction of the panel, are each formed between adjacent light absorption layers. In the shadow mask, the plural electron beam passage apertures are arranged in lines to form aperture rows each extending in the short-axis direction of the panel, and a plurality of aperture rows each including a plurality of electron beam passage apertures are arranged in parallel in the long-axis direction.
In the color cathode-ray tube described above, generally, three electron beams need to be correctly landed on their corresponding three-color phosphor layers of the phosphor screen through the apertures of the shadow mask in order to display an image on the phosphor screen without color purity drifts. It is thus necessary to exactly hold the shadow mask in a given position with respect to the panel.
A color cathode-ray tube, which has been improved in viewability by decreasing the curvature of the outer surface of an effective section of a panel nearly to that of the plane, has recently been put to practical use. In this color cathode-ray tube, the curvature of the inner surface of the effective section also needs to decrease in view of formability and viewability of the panel, as does the curvature of a surface opposed to a phosphor screen of a shadow mask in accordance with the curvature of the inner surface of the effective section.
If, however, the curvature of the inner surface of the panel decreases, a reduction in the atmospheric-pressure resistance of the vacuum envelop will turn into problems. If the effective section totally increases in thickness in order to avoid the reduction, the transmittance of the panel decreases and consequently the brightness of an image displayed through the effective section lowers.
If the peripheral portion of the effective section is formed thicker than the central portion thereof in order to secure the atmospheric-pressure resistance, a difference in transmittance between the central and peripheral portions becomes wider in accordance with the increase in the thickness of the peripheral portion. This difference makes a large difference in brightness between the central and peripheral portions of the panel and greatly decreases the viewablity of the panel when an image is displayed. If the transmittance of the panel is increased to avoid these, the contrast of the image is degraded. A method of adhering a film of low transmittance onto the outer surface of the effective section can be adopted as a measure for avoiding the degradation of contrast; however, in this case, the number of manufacturing steps increases and so does the manufacturing costs.
If the shadow mask decreases in curvature in accordance with the inner surface of the effective section of the panel, it decreases in mechanical strength and is easily deformed in the manufacturing process of the color cathode-ray tube. The deformation of the shadow mask degrades color purity.
Furthermore, the color purity is degraded by doming of the shadow mask. In the color cathode-ray tube, as operation principles, electron beams reaching the phosphor screen through the apertures of the shadow mask are not more than one-third of all electron beams emitted from the electron gun, and the other electron beams collide with the portion of the shadow mask other than the apertures and heats the shadow mask. By this heating, the shadow mask is thermally expanded to cause doming expanded in the direction of the phosphor screen.
The doming varies a distance (q value) between the phosphor screen and the shadow mask. If the variation exceeds a permissible range, the electron beams land on positions displaced from target three-color phosphor layers and thus the color purity is degraded. The displacement of the landed beams due to the doming varies with the brightness of an image pattern to be displayed and the duration thereof. When a high-brightness image pattern is locally displayed, a local doming occurs and displaces the landed beams locally in a short time. This doming appears more conspicuously when the shadow mask decreases in curvature and turns into unavoidable problems when the effective section of the panel is flattened.
The present invention has been developed in consideration of the above problems and its object is to provide a color cathode-ray tube which is improved in image quality by reducing a deterioration in color purity and a difference in brightness, without degrading the atmospheric-pressure resistance of a panel or the mechanical strength of a shadow mask, even when the outer surface of an effective section of the panel is flattened.
To attain the above object, a color cathode-ray tube according to one aspect of the present invention comprises:
an envelope including a panel having a substantially rectangular effective section with a substantially flat outer surface, and a funnel joined to the panel;
a phosphor screen formed on an inner surface of the panel, the phosphor screen including a plurality of strip-shaped light absorption layers arranged in parallel with one another and a plurality of strip-shaped phosphor layers arranged in parallel and each formed in a gap between adjacent light absorption layers;
an electron gun arranged in a neck of the funnel, for emitting electron beams toward the phosphor screen; and
a shadow mask provided opposite to the phosphor screen, the shadow mask having a plurality of aperture rows arranged in parallel, each of the aperture rows including a plurality of apertures arranged in line, a bridge being interposed between adjacent apertures,
wherein the envelope includes a tube axis extending through a center of the effective section and the electron gun, a long axis crossing the tube axis at right angles, and a short axis crossing the long axis and the tube axis at right angles;
the effective section of the panel is formed such that a corner portion is 8 mm to 15 mm thicker than a central portion, and transmittance of the central portion is set to 40% to 60%; and
the phosphor screen is formed such that a ratio of a width of each of the light absorption layers to the pitch of the phosphor layers is larger in the central portion of the effective section than in at least a part of a peripheral portion thereof.
In the color cathode-ray tube having the above structure, even though the outer surface of the effective section of the panel is almost flattened, an adequate atmospheric-pressure resistance can be maintained in the vacuum envelope and the peripheral portion of the panel can be prevented from decreasing in brightness.
According to the color cathode-ray tube of the present invention, the phosphor layers and the light absorption layers of the phosphor screen, and the aperture rows of the shadow mask extend substantially in parallel with the short axis, and if a width of apertures formed in a central portion of the shadow mask along the long axis is Ac, a width of apertures formed in short axis end portions thereof along the long axis is Av, a width of apertures formed in long axis end portions thereof along the long axis is Ah, and a width of apertures formed at each corner portion thereof along the long axis is Ad, following relationships are given:
Acxe2x89xa6Av less than Ad, and Ac less than Ahxe2x89xa6Ad.
Moreover, according to the color cathode-ray tube of the present invention, the aperture rows of the shadow mask extend substantially in parallel with the short axis, and if a pitch of apertures arranged in a central portion of the shadow mask along the long axis is Wc, a pitch of apertures arranged in a short axis end portions thereof along the long axis is Wv, a pitch of apertures arranged in a long axis end portions thereof along the long axis is Wh, and a pitch of apertures arranged at a corner portion thereof along the long axis is Wd, following relationships are given:
Wcxe2x89xa6Wv, and 1.3Wc less than Wh less than Wd.
In the color cathode-ray tube having the above structure, the pitch of rows of electron beam passage apertures in the peripheral portion of the panel is 1.3 times or larger than that in the central portion thereof. It is thus possible to obtain an adequate margin for multicolor emission of electron beams caused by variations in electron-beam landing position due to thermal expansion of the shadow mask can be obtained, thereby suppressing a degradation in color purity due to the variations in electron-beam landing position.
In the color cathode-ray tube of the present invention, if thicknesses of a central portion, long axis end portions, short axis end portions, and corner portions of the effective section of the panel are tc, tv, th, and td, respectively, it is desirable to satisfy the following relationships in order to improve in strength:
xe2x80x83tc less than tv less than td, and tc less than th less than td.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.